Wireless telecommunications systems are divided into a series of cell areas covering a service area. Each cell area has a transmitting base station using an operating frequency set comprising a plurality of radio channels to communicate with mobile subscribers. Each channel represents an information signal at a particular frequency carrier or band.
It has been found advantageous to combine these channels for transmission purposes. The channels are all combined by a broadband signal combiner into a multi-channel signal at low power levels and then amplified by a single linear amplifier (or its equivalent a plurality of linear amplifiers in parallel, each amplifying a reduced power version of the same multi-channel signal) to raise the multi-channel signal to an appropriate transmit power level.
The various radio channels are distributed in frequency with respect to each other in that each operates within a different frequency band. Each channel is FM modulated and hence has a signal of substantially constant amplitude. The peak occurrences of the combined signal is a highly complicated function of the individual carrier frequencies, modulation methods, signal contents and noise. Since the simultaneous occurrence of individual signal peaks can not easily be avoided, a multi-channel signal is subject to power maximums where the peak power significantly exceeds the average power of the envelope due to constructive addition of the individual signals.
It has been observed that, for narrowband FM, the modulation in some systems is not of adequate magnitude to significantly randomize the envelope. That is, the envelope can settle into a repetitive state of high peaks.
A straight forward response to this problem has been to select the power rating of the linear amplifier to accommodate the theoretical maximum peak power level of the composite multi-channel signal. Since signals add as voltage vectors the maximum peak power level of the composite multi-channel signal is N*N times the power of a single carrier signal where N is the number of signals combined to produce the the composite multi-channel signal. However, this significantly increases the cost of the linear amplifier since its power rating must be increased in proportion to the square of the number of signals processed. This high power rating is also only needed for a small fraction of the operating time of the amplifier (that is, during the high peaks of the combined multi-channel signal). It is most desirable to operate the system with amplifiers of power handling ratings based on the average power of the sum of the carriers than to require that an amplifier be rated to handle the high peak power caused by the constructive addition of the individual carriers.